Polyamides are commonly employed for coating metal substrates, especially because of their good mechanical properties such as abrasion resistance, impact strength and chemical inertness towards many products such as hydrocarbons, bases and inorganic acids.
The adhesiveness of polyamides to metals is known, however, to be insufficient, as a result of poor wetting ability of polyamides in a molten state. To overcome this shortcoming the metal substrate has been coated with an undercoat, called an adhesion primer, intended to ensure the mechanical bonding and anchoring of the polyamide powder. The adhesion primer which is employed is generally based on heat-curable resins and supplied in powder form or in solution or in suspension in organic or aqueous solvents. Additional plant must therefore be provided for the possible removal of the solvents and for curing the primer before the substrate which is thus clad is coated with the polyamide powder.
In addition, the curing and/or drying of the primer significantly lengthen the duration of the coating operations and hence their cost.
Mixtures of polyamides and of epoxy/sulphonamide resins which can be employed for coating metal substrates without employing an adhesion primer undercoat have been described in Patent EP 0 412 888. These mixtures of polyamide and of epoxy/sulphonamide resins in powder form can be applied to the substrate with an electrostatic gun. It then suffices to pass the substrate thus coated through an oven to obtain melting of the powder and a uniform coating. It is also possible to preheat the substrate above the melting point of the powder and to immerse it in a fluidized bed of the powder.